I think his original "plan" was to make them all disappear via the tanks (physical degradation over time would probably have achieved that). He'd just claim that SW found out about his affairs, confronted him, and then took she off with the girls.
But SW's flight delay, NU and his mother-in-laws persistence which caused LE getting involved quickly, put a major crimp in his "plans". He ran out of time and had to make something up...SW strangling her girls, and him strangling her.
He probably ran into complications disposing of SW's body (tank opening too small/too hard to carry her?) and thought he's temporarily hide her, then come back later and do a better job of disposal.
Makes me sick to type those words; disposal, degradation, disappear. :-(
Normal people will never be able to comprehend how a spouse and parent can do those things and think that way. Normal people have empathy and the ability to truly love. I don't believe he does.